This invention relates to PAR (parabolic aluminized reflector) lamps and, in particular, to means for attaching a filter over the lens of such lamps.
In the prior art, a variety of proposals have been made for providing a filter for a PAR lamp. Typically, the filter is a color filter, although other filters have been used in the prior art and can be used in the present invention.
One type of color filter utilizes a stained or dyed lens, made for example by ion implanting copper in the outer surface of the lens. However, staining the glass lens is a difficult process to control. Also, as the light transmission of the filter is increased, the color is generally degraded and vice versa. When governmental limits on wattage are added to the problem, the result is that only a small percentage of some lamp types meet all SAE (Society of Automotive Engineers) specifications for color and photometry. Alternatively, internal glass filters are costly and difficult to assemble into the lamp.
Prior art attempts at solving the problem include coating the lens with a transparent plastic and using a separate plastic filter, fluted or unfluted, in front of the lens. While the former approach provides an integral package, there are problems with variations in coating thickness. Also, solvents and spraying and drying equipment are required. With the latter approach, customer costs are higher and there may be increased transmission loss due to dirt on the inner surface of the filter. Also, at present, some state vehicle codes prohibit this type of construction.